The present invention relates to a control system and method for reducing the NOx emissions from a multiple-intertube pulverized-coal burner. More specifically, the present invention relates to a control system and method for use with the combination of a known NOx emission reduction system and an improved means of detecting and monitoring the true mass flow of pulverized coal through each burner delivery pipe. The control system and method allows for better control of the burner stoichiometry and, therefore, a further reduction of NOx emissions from such a burner.
NOx refers to the combination of nitric oxide (NO) and nitrogen dioxide (NO2) gases, which may be produced during the burning of coal when nitrogen is released from coal particles in the presence of excess oxygen. Both NO and NO2 are classified as pollutants under the Clean Air Act and, therefore, a reduction in the emission thereof is highly desirable.
In an electric power plant, multiple-intertube pulverized-coal burners commonly form part of a roof-fired boiler that is used to generate steam for driving electrical energy-producing generators. Such a roof-fired boiler will typically have more than one multiple-intertube pulverized-coal burner. The multiple-intertube pulverized-coal burners typically utilize a series of coal pulverizing mills that pulverize larger pieces of coal into much smaller particles. These coal particles are then carried by a primary air supply through a plurality of coal supply pipes to the multiple-intertube pulverized-coal burners for subsequent combustion within a combustion chamber. As mentioned previously, during the combustion process NO and NO2, as well as other undesirable gases, may be produced. However, it has been found that the amount of these gases produced can be reduced by better controlling the stoichiometry of the combustion process.
A method and apparatus that may be retrofitted to existing multiple-intertube pulverized-coal burners to provide such a reduction through improved stoichiometry has been previously disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183, all of which are incorporated by reference herein. In U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183, the method and apparatus controls the amount of both secondary and interjectory air to better regulate the combustion process.
The method and apparatus disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723, and 6,155,183 controls the amount of secondary air and supplies the secondary air to the burners in an internal two-stage process. The first stage includes secondary air dampers and air flow stations that regulate the amount of secondary air to the burners. A portion or balance of the required secondary air is directed through hot air ducts to interjectory air plenums located along the furnace front wall. The secondary air flowing directly to the burners is baffled to provide a low velocity, fuel-rich central core for combustion of the fuel""s volatile component in a reducing environment. The periphery of the burner maintains an oxygen-rich boundary layer that protects against reducing environments along waterfalls and corrosion potentials, and provides sustained combustion of the fixed carbon. The second stage of the process then uses one modulating interjectory air port per burner to provide the balance of the required total combustion air and sufficient turbulence to complete the combustion process. This two-stage process provides for a precise measurement of both secondary and interjectory air to the burners at all times, allowing enough combustion air to support both the burning of the fuel""s volatile component and the fixed carbon, while limiting the supply of excess oxygen to reduce the potential of the fuel-bound nitrogen released with the burning of the volatile component and atmospheric nitrogen from being converted to NOx.
In order to attain the most ideal possible burner stoichiometry by the above apparatus and method, it is necessary to accurately control the air/fuel ratio in each individual burner. Unfortunately, because a single coal pulverizing mill may serve multiple burners through multiple supply pipes, it has been difficult in the past to obtain the true mass flow of coal to a given burner and, thus, an accurate air/fuel ratio. One method of determining the mass flow of coal has been to measure the amount of bulk coal entering each pulverizer and to divide that amount by the number of delivery pipes connected thereto. Alternatively, a flow sensor may be employed to measure the mass flow leaving the pulverizer and the measurement thus obtained may be divided by the number of delivery pipes to calculate a theoretical mass flow through each pipe. However, due to dimensional differences from pipe-to-pipe, clogging or build-up in certain pipes but not others, and because of numerous other variants that can cause the flow of coal through one supply pipe to differ from that of the next, the assumption of equivalent mass flow of coal to each burner is erroneous and has often led to a less than optimum air/fuel ratio within a given burner.
The proper combustion process for coal requires approximately 7.5 pounds of air for every 10,000 BTU of coal. Although the BTU rating for coal used in a typical multiple-intertube pulverized-coal burner may vary from approximately 9,500 BTU/pound to approximately 12,500 BTU/pound, the actual BTU rating of the coal used at any particular location, at any given time, is generally known from testing. Thus, by accurately determining the mass flow rate of pulverized coal through a particular supply pipe, a proper amount of combustion air can be supplied to the burner to which the supply pipe is connected, and the burning of the coal can be optimized.
Sensors have been developed that may be used to accurately determine the mass flow rate of a substance, such as coal, through a conduit. Certain of these sensors use electrodes to measure the electrostatic charge of the particles traveling through the conduit; others monitor microwave absorption, attempt to directly measure air flow with and without particles entrained therein, measure the impact of particles, or measure the travel time of ultrasonic signals. Another type of sensor can measure the mass flow of coal particles by generating an alternating electric field within a feed pipe, measuring the attenuation of the electric field, and employing a derivation process to determine the quantity of solids in the flow. This type of sensor has proven particularly amenable to use with the present invention, and an exemplary device is disclosed in U.S. Pat. No. 6,109,097.
What is needed and has been heretofore unavailable, however, is a system employing a combination of the emission reduction method and apparatus disclosed in U.S. Pat. Nos. 5,771,823, 5,960,723 and 6,155,183 with an accurate means of determining the mass flow rate of pulverized coal through each burner supply pipe, such as the sensor disclosed in U.S. Pat. No. 6,109,097. In this manner, an optimum reduction in NOx emissions from a multiple-intertube pulverized-coal burner may be realized. The present invention contemplates such a system, and more specifically a control system and method for allowing the successful operation of such a system.
To improve the performance of a NOx emission reducing roof-fired boiler, the control system and method of the present invention is able to control the technology disclosed by U.S. Pat. Nos. 5,771,823, 5,960,723 and 6,155,183, while also incorporating input data from a mass flow sensor located in each coal supply pipe, preferably a sensor such as that disclosed in U.S. Pat. No. 6,109,097. The mass flow of coal through each burner supply pipe is measured and summed to determine the total mass flow of coal to the furnace. The ratio of coal flow through a given burner pipe to that of the average for all burner pipes on in service burners is also determined. The steam flow for the boiler is also known, and is used to determine the total theoretical combustion air required at each burner, including primary, secondary, and interjectory air. This combustion air demand may then be corrected based on the actual coal flow to each burner so that the stoichiometric ratio of the combustion air and fuel (coal) is properly maintained, thereby producing less pollutants.